1. Field of the Invention
This invention relates to an intake system for an internal combustion engine, and more particularly to an intake system for an internal combustion engine having a bypass air control system for controlling the amount of auxiliary air to be introduced into the combustion chamber bypassing the throttle valve in the intake passage.
2. Description of the Prior Art
As disclosed in Japanese Unexamined Patent Publication Nos. 54(1979)-98413 and 59(1984)-162340, for instance, there has been known an intake system for an internal combustion engine having a bypass air control system, that is, an intake system which is provided with a bypass passage bypassing the throttle valve in the intake passage to feed auxiliary air to the engine during idling, a control valve for opening and closing the bypass passage and a control means for controlling the control valve according to the operating condition of the engine such as the engine temperature, the engine load and the like, and in which the amount of air-fuel mixture to be introduced into the engine is controlled to control the engine speed according to the operating condition of the engine, thereby effecting feedback control of the idling speed, correction of the idling speed according to load on the engine or the like.
The following problems are in the intake system having such a bypass air control system. That is, the output characteristics of the control valve, i.e., the relation of the electric current for driving the control valve to the amount of air flowing through the bypass passage, is nonlinear as shown in FIG. 9. That is, the output characteristic curve of the control valve has a relatively small inclination at the marginal parts where the duty are near 0% or 100% and has a relatively large inclination at the intermediate part, the intermediate part being substantially linear. Therefore, the change in the amount of auxiliary air for a given change of the duty of the control valve varies depending on the original position of the control valve. This adversely affect the precision of control. The original position of the control valve changes with time or depending on whether the engine is loaded. If the control is effected using only the intermediate part of the output characteristic curve of the control valve in order to avoid the problem, useful range of the control valve is remarkably narrowed for the capacity thereof, and the engine is apt to stall due to disturbance. Otherwise, the control valve must be very large in capacity.